The present invention relates to a one-pack coating composition for coating motor cars and the like. In particular, the present invention relates to a one-pack coating composition having an excellent io storability, being capable of forming a coating free from unnecessary coloring and having an excellent weather resistance.
A curing resin composition having a siloxy group, an acid anhydride group, an epoxy group and an alkoxysilyl group as main functional groups is well known as a one-pack coating composition excellent in acid resistance and resin stability [see, for example, Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. Hei 3-172318].
A curing catalyst for accelerating the curing reaction of the carboxyl group derived from an acid anhydride and an epoxy group is usually used for such a curable resin composition to accelerate the curing reaction of the oligomer or polymer having the above-described functional groups. However, when such a curing catalyst is previously incorporated into the paint composition, problems are caused. Specifically, the carboxyl group is formed from the acid anhydride group by water (moisture) in air during the storage to cause the curing reaction with the epoxy group; the curing reaction of the acid anhydride group and the epoxy group proceeds; the carboxyl group thus formed acts as the catalyst to remove the blocking group from the blocked hydroxyl group; and the viscosity of the coating composition gradually increases during the storage. Therefore, recently, a so-called heat-potential curing catalyst which dissociates upon heating to act as a curing catalyst has come to be used (see, for example, J. P. KOKAI No. Hei 5-271526).
On the other hand, it is well known that a light stabilizer, antioxidant, ultraviolet absorber, etc. are incorporated into the coating composition for the purpose of improving the weather resistance thereof.
However, it has been found that, when the light stabilizer, antioxicant, ultraviolet absorber, etc. are incorporated into the coating composition containing the heat-potential catalyst, the composition cannot be used as a one-pack coating composition, since the viscosity of the coating composition is gradually increased during the storage or an unnecessary coloring of the coating film is caused depending on the kind of these additives to be used.